1. Field of the Invention
The present invention relates to a method for calibrating a pyrometer, a method for determining the temperature of a semiconducting wafer and a system for determining the temperature of a semiconducting wafer.
2. Description of Related Art
Measurements of the temperature of a semiconducting wafer belong to the most important tools during quality control in the semiconductor production.
In many applications, the semiconductor production is realised within a processing chamber and the temperature of the semiconducting wafer which is processed inside the processing chamber is contactlessly measured by use of a pyrometer which is adapted to intercept and measure the thermal radiation which is emitted by the semiconducting wafer. Said thermal radiation is then used to calculate the temperature of the semiconducting wafer. Due to the high temperatures inside the processing chamber, the pyrometer is advantageously located outside the processing chamber and said thermal radiation passes through a window of the processing chamber before entering the pyrometer.
However, the geometry inside the processing chamber may change for different applications, for example due to different sizes or positions of semiconducting wafers inside the chamber. Such different geometrical parameters may result in different outcoupling characteristics of the thermal radiation from the semiconducting wafer to the pyrometer through the window of the processing chamber. Also, the optical transmission characteristics of the window of the processing chamber itself may change, for example due to process related window coating. Such optical transmission characteristics may also result in different outcoupling characteristics of thermal radiation from the semiconducting wafer to the pyrometer through the window of the processing chamber. A variation of the outcoupling characteristics may however severely affect the accuracy of the determination of the temperature of the semiconducting wafer. Therefore it is necessary to re-calibrate the pyrometer before starting the temperature measurement, even if it has been carefully pre-calibrated on a separate calibration stage (i.e., a black-body reference light source) before.
A calibration process in the sense of the present invention is realised in that prior to the manufacturing process—including measurements of the temperature of a semiconducting wafer during manufacturing—a pyrometer is calibrated by providing a calibration sample inside the processing chamber. The calibration sample is a sample which preferably corresponds to a semiconducting wafer in terms of size and material, where said semiconducting wafer is intended to be manufactured after the calibration process. Therefore, once the calibration sample is heated to a known temperature value or to a plurality of known temperature values, said calibration sample can accurately simulate a thermal radiation which would be emitted by the real semiconducting wafer when being manufactured inside the processing chamber. The (so simulated) thermal radiation can be measured by the pyrometer which is located outside the processing chamber and the measured thermal radiation is then assigned with a certain temperature value or certain temperature values. Once, such assignment (also referred to as calibration) has been carried out, a semiconducting wafer can be manufactured inside the processing chamber and the temperature can be exactly derived from the thermal radiation measured by the pyrometer (which is located outside the processing chamber) during the manufacturing process because the measured thermal radiation can be re-assigned with accurate temperature values.
A conventional method for calibrating a pyrometer is described in more detail in connection with FIGS. 1 and 2. In order to carry out the above described calibration process it is necessary to exactly know the temperature of the calibration sample 12 without locating the pyrometer inside the processing chamber, so that the measured thermal radiation can be assigned with an accurate temperature value. Therefore, as illustrated in FIG. 1, the calibration sample 12 comprises a semiconducting wafer 2 such as a silicon substrate and a thin aluminium layer 3 disposed thereon. Further, a pyrometer 1 is provided which is adapted to intercept and measure thermal radiation and which is furthermore adapted to perpendicularly irradiate an optical radiation having a predetermined wavelength onto the calibration sample 12. During heating of the calibration sample 12, an eutectic reaction starts at exactly 577° C. where aluminium starts to diffuse into the silicon layer 2. Before the calibration sample 12 reaches a temperature of 577° C., the calibration sample 12 comprises a high reflectivity due to the thin aluminium layer 3. However, after reaching a temperature of 577° C., the aluminium layer completely diffuses into the silicon layer 2 and therefore the calibration sample 12 comprises a relatively low reflectivity. That is, the reflectivity significantly decreases at 577° C. which can be observed from outside of the processing chamber. Furthermore the pyrometer 1 measures the thermal radiation (besides the reflection signal) which is emitted from the calibration sample 12 and calculates the temperature of the calibration sample 12 using the measured thermal radiation. Once, the pyrometer has not been calibrated before—such as shown in FIG. 2—the pyrometer shows a nominal temperature of e.g. 500° C. while the reflectivity/reflectance significantly decreases (at 577° C.). Therefore, the thermal radiation which is measured at that time is assigned (calibrated) with the exact temperature of 577° C. Due to the fact that the temperature characteristics are exactly given by the Planck function, the complete temperature range of the pyrometer 1 can be calibrated using this single reference point of 577° C.
However, the above described conventional method of calibrating a pyrometer comprises the following disadvantages. The reaction characteristics of the eutectic reaction strongly depends on the thickness of the aluminium layer which makes it difficult to exactly determine the temperature of 577° C. from the reflection signal. Furthermore, the calibration sample comprises different heating characteristics than an identical silicon wafer without the aluminium layer (which is used during manufacturing) due to the different metallic radiation absorption inside the processing chamber. Furthermore the reaction characteristics of the eutectic reaction depend on the slope of the temperature ramping, thereby making it difficult to exactly determine the temperature of 577° C. from the reflection signal. Furthermore the reaction characteristics of the eutectic reaction depends on the area coverage ratio of the aluminium layer on the silicon wafer. Furthermore, the Al—Si eutectic point at 577° C. is relatively low and therefore methodical errors of the pyrometer calibration are amplified for high temperature processes (up to 1300° C.) resulting in temperature errors up to 30 K. More importantly, the calibration sample can be used only once because the aluminium layer is destroyed during the calibration process.
It is therefore an object of the present invention to provide a method for calibrating a pyrometer which overcomes the disadvantages of the prior art. It is another object of the present invention to provide a method for determining the temperature of a semiconducting wafer and a system for determining the temperature of a semiconducting wafer which comprise a more accurate, more reliable and more cost-effective calibration process.